1. Field of the Invention
This invention relates to a failure detection system of a pressure sensor for detecting pressure in a fuel tank or a passage connected to a fuel tank.
2. Description of the Related Art
An evaporation fuel processing unit for storing evaporation fuel occurring in a fuel tank in a canister and supplying an inlet system of an internal combustion engine at the appropriate time is widely used. If leakage occurs in the evaporation fuel processing unit, evaporation fuel is released into the atmosphere. Thus, a pressure sensor is provided for detecting pressure in the evaporation fuel processing unit and leakage occurring in the fuel tank or the canister is detected according to a detection value of the pressure sensor.
If the pressure sensor itself fails, accurate leakage detection cannot be executed. Therefore, a technique has been known such that the pressure sensor is determined to fail, when the detection value of a pressure sensor becomes a value outside a predetermined range of upper and lower limit values (for example, range of atmospheric pressure .+-.45 mmHg) just after cold start of an engine and the state continues for a predetermined time.
However, such a technique cannot detect a failure occurred in a state where a pressure sensor output varies continuously for some reason, for example, a failure such that output waveform as indicated by the solid line in (i) of FIG. 4 is produced. Therefore, leakage cannot be detected accurately in a state where such a failure occurs.